KR900008599B1 - Multimode fiber optic rotation sensor - Google Patents

Abstract

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Description

Multimode Fiber Optic Rotation Sensor

Figure 1 detects the phase difference between a single continuous optical fiber strand to which light rays from a light source are coupled, a multimode sense loop formed from this single continuous strand, and also the light waves propagating in the reverse direction through the optical fiber loop. A schematic diagram of a rotation sensor of the present invention, showing a detection system for the purpose of doing so.

2 is a schematic diagram of a conceptual model of an optical fiber loop 16 showing the electric field component of this backward propagation wave as it passes through the optical fiber loop, for an exemplary mode pair.

FIG. 3 is a schematic diagram of the conceptual model of FIG. 2 showing the electric field component of the backward wave after the backward wave passes through the optical fiber loop.

FIG. 4 shows a vector of the real axis representing the vector sum of the direct current terms by the electric field components shown in FIG. 3, and the vector sum of the interference terms caused by the electric field components shown in FIG. Another vector is shown, and also the response of the vector representing the interference term for phase error caused by (1) rotationally induced Sagnac phase difference and (2) nonrotationally induced phase difference. Vector diagram of output signal.

FIG. 5 is a graph corresponding to the vector diagram of FIG. 4 showing the relationship between the Sagnak phase difference versus the optical intensity measured by the detector, showing the effect of nonrotating induced phase error.

6 is a vector diagram of an interference term resulting from the electric field components of group III.

FIG. 7 is a vector showing a vector sum of two vectors shown in FIG. 6 and a phase error associated with the vector.

FIG. 8 is a vector diagram showing the case where the vectors of FIG. 6 have the same size. FIG.

FIG. 9 is a vector diagram of a composite vector representing the vector sum of the vectors of FIG. 8, showing that phase errors can be eliminated by making the magnitude of the vector the same.

FIG. 10 is a schematic view showing a portion of an optical fiber extending between the couplers of FIG. 1, the filter comprising a transfer hologram inserted between the couplers.

FIG. 11 is a schematic diagram illustrating a technique for making the transport holographic filter of FIG. 10. FIG.

* Explanation of symbols for main parts of the drawings

10: light source 11, 17 and 19: optical fiber

12: first directional coupler 14: second directional coupler

16: loop 20: photodetector

22 detection circuit 24 lock-in amplifier

26: signal generators 28 and 40: phase modulator

70: filter 80 and 82: optical fiber

84: holographic plate.

The present invention relates to an optical fiber rotation sensor, and more particularly, to an optical fiber rotation sensor using a Sagnac interferometer having a multimode optical fiber sensing loop.

This patent application is a continuation patent application of US Patent Application No. 318,813, filed November 6, 1981, entitled "Multimode Optical Fiber Rotational Sensor."

Fiber optic rotation sensors typically include a loop of fiber optic material that is coupled such that light waves propagate in opposite directions around the loop. When the loop rotates, a phase difference is generated between the reverse propagation waves according to the known "Sagnak effect", and the magnitude of the phase difference corresponds to the rotation speed of the loop. When the backward propagation waves are recombined, these backward propagation waves constructively or destructively interfere with each other, thereby generating an optical output signal whose intensity varies according to the rotational speed of the loop. Rotational sensing can be achieved by detecting this optical output signal.

In order to accurately detect the rotationally induced Sagnak phase difference, the non-rotationally induced phase difference caused by the physical properties of the optical fiber (e.g., birefringence of the optical fiber) must be substantially eliminated, because this phase difference This is because a phase error appears in the optical output signal because it cannot be distinguished from the phase difference. The interferometer is referred to as "reversible" when the optical paths of the reverse propagation are the same when the loop is at rest and the non-rotatively induced phase difference is eliminated, or the interferometer is "irreversible" if these paths are not identical. It is called.

The irreversibility of an optical fiber interferometer type rotation sensor is caused by two factors. First, the optical fiber can hold several different basic modes (e.g., HE ₁₁ mode, TE ₁₀ mode, etc.), since each mode has a different propagation rate or phase velocity (in this case, one Mode can be seen as a specific optical path through the optical fiber). Second, the birefringence of the optical fiber is not uniform according to the optical fiber, and therefore there is a coupling of light energy between the modes. If these two factors are present, each reverse propagation wave will travel to a different optical path around the optical fiber loop, so that when the reverse propagation waves recombine, there is a phase difference between them. This outlier, which can be several orders of magnitude larger than the Sagnak phase difference, is indistinguishable from the rotationally induced Sagnak phase difference, and thus appears to be an error in the optical output signal itself. At this time, it is important that the two factors do not exist in each case to destroy reversibility. Both of these factors must be present in order for the irreversibility to appear.

In the prior art, efforts have been made to satisfy reversible conditions by using a single mode optical fiber having only one basic transfer mode (ie HE ₁₁ mode). The use of single-mode optical fibers would theoretically indicate reversibility by eliminating the first factor, but in practice a single-mode optical fiber had two orthogonal polarization modes (ie, orthogonal polarization modes with slightly different propagation phase velocities). I found that This difference in propagation speed is very small, but is sufficient to cause irreversible operation of a single mode fiber optic rotation sensor. This problem is described, for example, in "Optics Letters", Volume 4, page 152 (April 1979) by R. Ulrich and M. As described by M.Johnson as "Fiber Ring Interferometer Polarization Analysis," an optical fiber polarizer is used to detect one of the two polarizing modes of a single mode optical fiber. It was solved in the prior art by blocking.

However, the use of multiple mode optical fibers for sensing rotation is of little interest because of the large number of modes in the multiple modes and the different propagation speeds. The use of graded index multimode optical fibers, rather than step index multimode optical fibers, can reduce the difference in propagation speed between the modes, but in these two types of optical fibers Since the difference in propagation speed is quite large, multimode optical fibers have been considered to be unsuitable for use in rotational sensors. The problem is also the fact that a series of mode patterns (referred to herein as "generalized polarization modes") with nearly identical but not exactly the same propagation velocity exist within each basic mode of the multimode optical fiber. Further complicated by. The difference in propagation speed between generalized polarization modes within a particular base mode is typically of the same order and magnitude as the difference in propagation speed between the base modes of the gradient index multimode optical fiber. Due to the large number of modes in the multimode (thousands in some cases) and the resulting propagation speed issues, it has so far been impossible or at least impractical to achieve reversibility in a Sagnak interferometer detector using one multimode optical fiber. Has been recognized.

The rotation sensor of the present invention comprises one continuous strand of multimode optical fibers forming a loop in the form of a Sagnak interferometer. In addition, an optical fiber directional coupler is used to close the loop and couple the pair of backward propagation waves to the loop. The combiner recombines the reverse propagation wave passing through the loop to form an optical output signal, which is applied to the photodetector. Rotating the loop creates a phase difference between the reverse propagation waves, which changes the magnitude of the optical output signal according to the rotational speed. By detecting the optical output signal, the rotational speed can be known directly.

The phase error in the optical output signal exhibited between the reverse propagation waves by the non-rotationally induced phase difference uses (1) light rays with an interference length shorter than the difference in the optical path length between the propagation modes, and (2) By using a detector large enough so that all optical output signals are applied to the detector surface, and (3) combining light rays into the multimode optical fiber such that the field amplitude in each propagation mode is approximately equal, it can be systematically reduced or eliminated. Reducing or eliminating the phase error in this way can detect the rotationally induced sagnak phase error, thereby obtaining a practically useful multi-mode rotation sensor.

The multimode optical fiber rotation sensor of the present invention is surprisingly stable and therefore relatively insensitive to changes in optical fiber birefringence exhibited by peripheral factors such as temperature or vibration. Since the phase change and coupling between the modes due to the change of the optical fiber birefringence is leveled across the modes, the overall effect of the peripheral factor on the optical output signal is very small. Since the stability of the optical output signal is a function of the number of modes, it is desirable to use optical fibers that hold a large number of modes. At this time, since the stepped index multi-mode optical fiber can hold more modes than the equivalent sized gradient index optical fiber, it is preferable to use the stepped index multi-mode optical fiber in the present invention.

The multimode optical fiber rotation sensor of the present invention has an advantage over the conventional single mode optical fiber rotation sensor. One of the most important advantages is that manufacturing costs are lower because multimode fiber and multimode components (eg, combiners) can be processed more easily and at a lower cost than single mode fiber and its components. will be. In addition, since spatially incoherent light rays can be used in the present invention, in the rotation sensor of the present invention, an inexpensive light emitting diode (LED) can be used as a light source instead of an expensive laser having a long spatial interference length.

Another advantage is that the multimode optical fiber sensing loop is less affected by the Kerr effect, because in the present invention the larger effect is spatially leveled across a large number of modes. Another reason to be less affected is that the optical intensity in the optical fiber is reduced because of the large diameter of the core of the multimode optical fiber. In addition, since the typical birefringence of the optical fiber is larger than the birefringence induced by the Faraday effect, the multimode optical fiber is less sensitive to the Faraday effect which can be induced by an external magnetic field. Indeed, the linear birefringence of multimode optical fibers overwhelms the circular birefringence induced by the Faraday effect. And, a multimode rotation sensor can be manufactured as an entire optical fiber system using a continuous single strand of optical fiber material, as in the corresponding single mode detector.

In a second embodiment of the present invention, a modal filter including a transfer hologram is used to eliminate the non-rotationally induced phase error described above. The holographic filter is arranged to pass through the filter when the light beam from the light source is directed to the loop and then through the filter when returning to the detector, utilizing only one mode in the multimode optical fiber.

Hereinafter, with reference to the accompanying drawings will be described the present invention in more detail.

In the preferred embodiment shown in FIG. 1, the rotation sensor of the present invention includes a light source 10 for inserting CW light waves into a continuous single cycle or core of the multimode optical fiber 11. As used herein, the term " multimode optical fiber " means that the optical fiber is maintained in multiple base modes for a particular light source, which means that only one base mode is maintained for one light source. The term is opposed to fibers. The optical fiber 11 passes through the entrances A and C of the first directional coupler 12 and the entrances A and C of the second directional coupler 14. That is, the optical fiber 11 extends from the outlet C of the combiner 12 to the inlet A of the combiner 14 via the inlet A of the combiner 12 from the light source 10. The portion of the optical fiber 11 extending from the outlet C of the combiner 14 is wound around the loop 16. As one example, the loop 16 may consist of 1000 windings each having a cross section of about 150 cm 2. The end of the optical fiber 11 from the loop 16 passes through the entrances D and B of the combiner 14, the inlet D of the combiner 14 being formed towards the loop 16. . The small portion 17 of the optical fiber 11 exiting the outlet B of the combiner 14 ends non-reflectively without being connected.

The second optical fiber 19 passes through the entrances D and B of the combiner 12. The portion of the optical fiber 19 protruding from the outlet D ends non-reflectively without being connected. However, the portion of the optical fiber 19 protruding from the exit B of the combiner 12 is optically coupled to the photodetector 20 for generating an output signal proportional to the intensity of the applied light beam.

The invention also includes a detection electronics 22 consisting of a lock-in amplifier 24, a signal generator 26 and a phase modulator 28. As one example, the phase modulator 28 may be comprised of a PZT cylinder having a diameter of, for example, about 1 to 2 inches, such that a portion of the optical fiber loop 16 may be wound, for example 4 to 10 times. Since the optical fiber is adhered to the PZT cylinder 28 by a suitable adhesive, the optical fiber 11 is elongated when the cylinder 28 extends radially. At this time, the phase modulator 28 is driven by an AC modulated signal, which is provided on the conducting wire 30 from the signal generator 26 and has a frequency of, for example, 10 to 100 KHz. In order for the detection electronics 22 to operate properly, the phase modulator 28 is not on the center of the sense loop 16 but on one side of the loop 16, for example at the inlet D of the combiner 14. It is important that they be placed adjacently.

The AC modulated signal from the signal generator 26 is also supplied to the lock-in amplifier 24 via the lead 32. Lead 34 is connected to lock-in amplifier 24 to receive the output signal of detector 20. This amplifier uses the modulation signal from the signal generator 26 as a reference for operating the amplifier 24 to synchronously detect the output signal of the detector at the modulation frequency. Thus, the amplifier 24 effectively serves as a bandpass filter at the fundamental frequency (ie, the first harmonic frequency) of the phase modulator 28, thereby blocking all other harmonics of this frequency. For those skilled in the art, it will be readily understood that the magnitude of the harmonic component of the detector's output signal is proportional to the rotational speed of the loop 16 over its operating range. Since the amplifier 24 outputs a signal proportional to this first harmonic component, the amplifier 24 directly displays the rotation speed.

A more detailed description of the detection electronics 22 is published in International Patent Application No. PCT, published on October 14, 1982, publication WO 82/03456, entitled " Optical Fiber Rotation Sensor ", and used herein by reference. / US 82/00400. Such a detection system is also described in Optics Letters, Vol. 6, Issue 10 (October 1981) pages 502-504. Another detection system suitable for use in the present invention is described in J. S. 157, pp. 131-136. L. J.L. Davis and S. S. Ezekiel's article "Laser Inertial Rotation Sensors" (1978).

The shape and type of the multi-mode optical fibers used as the optical fibers 11 are not critical to the operation of the present invention. Therefore, multimodal optical fibers of either a stepped index or a sloped index can be used. However, the performance of the rotation sensor in terms of sensitivity to ambient influences such as temperature or vibration is shown as a function of the number of modes. Therefore, a stepped index multimode optical fiber that can hold more modes than an equally sized warp index optical fiber is desirable. In the illustrated embodiment, the optical fiber 11 is comprised of stepped index multimode optical fibers having a core diameter of about 50 microns, the optical fiber 19 being the same as this optical fiber 11.

Fiber optic directional couplers suitable as couplers 12, 14 are described in US Pat. No. 4,136,929, issued to Suzaki on January 30, 1979, which is incorporated herein by reference. As shown in Figures 3a-3d of this patent, the combiner consists of a pair of blocks having an arched groove to which each of the multimode optical fibers is mounted. The surface of these blocks is cut and processed so that the coating and a portion of the core are removed from one side of the optical fiber. Then, by placing the surfaces of the blocks oppositely, the exposed core portions of the optical fibers are placed in parallel. Preferably, the coupling ratios of the couplers 12 and 14 of FIG. 1 are set to 50%, respectively, so that the light rays inserted into the inlet A are evenly distributed to the outlet C and the outlet D. FIG. good. By replacing the coupler 12, 14 with a non-dividing machine, the rotation sensor of FIG. 1 can be constructed with a bulk optical component.

The light source 10 is important for the proper operation of the present invention. More specifically, the light source 10 provides light with high incoherence, so that the relative phases of the light rays in each mode of the optical fiber are random with respect to each other. In addition, the light beam is coupled to the optical fiber such that the amplitude of the electric field for each mode has the same magnitude. As described below, when these two conditions are met, certain types of phase errors induced non-rotationally are eliminated.

Suitable for use as the light source 10 is a surface emitting light emitting diode (LED) having a wavelength on the order of 700 to 900 nm. Since LEDs are long-term light sources, they fire each mode at the same intensity. In addition, the light rays generated by the LEDs are highly incoherent. In general, since LEDs are relatively cheaper than lasers, they are particularly preferred for use as light sources.

As another method, as shown by the dotted line in FIG. 1, by providing the phase modulator 40 adjacent to the light source 10, an incoherent light ray can also be generated using a coherent light source. The modulator 40 may be in the same form as the modulator 28, ie in the form of a PZT cylinder on which the optical fiber 11 may be wound. The modulator 40 may be driven by a signal generator (not shown) that generates a high frequency signal or a random signal higher than the detection bandwidth of the electronic circuit 22. However, the operating frequency of this signal generator should be different from the operating frequency of the signal generator 26. Also, the modulation must be done before reaching the loop 16, otherwise the rotation signal will be leveled to zero by this modulation.

In addition to providing incoherent light, modulator 40 may also serve as a mode scrambler to distribute the light evenly between the modes of the optical fiber. Basically, if the optical fiber is wound, for example 5 to 10 times, around a PZT cylinder of relatively small diameter (e.g. 1/2 to 1 inch), sufficient coupling is made between the modes so that the amplitude of the electric field between the modes This is substantially leveled.

The photodetector 20 is also important for proper operation of the rotation sensor. More specifically, the surface area of the photodetector must be large enough so that when the photodetector is disposed perpendicular to the axis of the optical fiber, all light rays emitted from the optical fiber 19 can be blocked by the photodetector. The diameter of the photodetector 20 is typically in the range of 2 to 10 mm, the exact size of which is the diameter of the multimode optical fiber 19, the numerical aperture of the optical fiber 19 [by this numerical aperture, the light is optical The degree of divergence of the light beam when exiting the fiber 19 is determined; and the distance between the end of the optical fiber 19 and the photodetector 20. In the illustrated embodiment, photodetector 20 is a standard PIN or Avalanche Silicon Photodiode having a diameter of 10 mm.

During operation, a continuous light wave (W _i) be the input from the light source 10 is transmitted through an optical fiber (11). When passing through the coupler 12, optical wave (W _i), a portion of the light beam (for example, 50%) is lost through the doorway (D). The remaining light will be transmitted from the gateway (C) of the coupler 12 to the coupler 14, two light waves (W _1), (W _2), a uniformly separated by a passage, then loop 16 is light waves (W ₁ ) And (W ₂ ) are recombined by the combiner 14 to form an optical output signal W ₀ . A portion of the recombined light wave W ₀ is lost through the inlet B of the combiner 14, and the remaining portion is transmitted from the inlet A of the combiner 14 to the inlet C of the combiner 12 and again. By separation, a portion (eg, 50%) is transferred to the optical fiber 19. The light wave W ₀ exiting from the end of the optical fiber 19 is imposed on the photodetector 20, thereby outputting an electrical signal proportional to the optical intensity of the light wave W0.

The intensity of this optical output signal varies in proportion to the form of light waves W ₁ , W ₂ (ie, constructive or destructive) and the amount of interference between these light waves W ₁ , W ₂ . Therefore, it becomes a function of the phase difference between the light waves W ₁ and W ₂ . At any moment, when the optical fiber 11 is "ideal" (ie, the birefringence of the optical fiber is uniform in the longitudinal direction), the intensity of the optical output signal can be measured so that the rotationally induced Sagnak phase difference can be accurately known. Thus, the rotation ratio of the optical fiber loop 16 can be accurately known.

As described above, conventional multimode optical fibers have (1) birefringence, and (2) this birefringence is not uniform in the longitudinal direction of the optical fiber, and thus the non-rotationally induced phase difference (ie, phase error) This phase difference is not "ideal" in that it generates a phase difference that is indistinguishable from the rotationally induced Sagnak phase difference. The present invention uses three different techniques to reduce or eliminate these phase errors, i.e. (1) non-coherence is such that the relative phases of the rays emitted to each mode of the optical fiber are random with respect to each other. Use a power source that generates high light beams, (2) make the field amplitude uniform for the light beams in each mode, and (3) use a detector with a large surface area to fertilize to capture substantially all optical output signal power. Is to use.

The deceleration or removal of such a phase error will be described in detail with reference to FIG. 2. FIG. 2 conceptually depicts any two modes of multimode optical fibers selected from any complete orthogonal mode set, thereby linearly superimposing the electric field of the orthogonal mode set, which describes the propagation pattern of the electric field in the multimode optical fiber. It can be explained as. It is assumed that the propagation speed of each mode is different from that of other modes. Also, to illustrate the fact that birefringence is not evenly distributed in the longitudinal direction of the optical fiber, it is assumed that there is a light energy coupling between the modes. Hereinafter, such an energy bond will be referred to as a "cross bond".

Indeed, multi-mode optical fibers may have thousands of modes, for example, but are described herein as having only two modes for convenience of description. However, the description of these two modes can be extended to the case of N modes, as will be described mathematically later.

The conceptual optical fiber model of FIG. 2 is used to represent the sense loop 16 (FIG. 1). The mutually opposite propagation waves W ₁ and W ₂ are shown to be coupled to the loop 16 by the combiner 14 as indicated by the dashed arrows. Two modes of the multimode optical fiber arbitrarily selected for the purpose of illustration include a first line connecting a pair of terminals C 'and (D') and a second pair of terminals C parallel to the first line. It is schematically shown by the second line connecting "), (D"). Terminals C 'and C "on the left side of FIG. 2 correspond to the entrance and exit C of the coupler 14, and terminals D' and D" of the right side of FIG. It corresponds to the entrance D. The above-described first and second lines connecting the terminals are used to denote arbitrary modes i and j of the optical fiber loop 16, respectively.

The crosslink between modes i and j is indicated by a pair of lines labeled "shunt 1" and "shunt 2", respectively. Shunt 1 represents the cross coupling between terminal C "and terminal D ', and Shunt 2 represents the cross coupling between terminal C' and terminal D". Although there is no bond between Shunt 1 and Shunt 2, the intersection 50 of the two Shunts 1 and 2 will be referred to as "bond center". In order to illustrate that the birefringence of the optical fibers is not uniform in the longitudinal direction so that they are not distributed symmetrically around the optical fiber loop 16, the coupling center 50 is shown to be offset from the center of the optical fiber loop 16. have. Thus, the path of the cross-linked light beam is longer in the path of one mode than the path in the other, so that a non-rotationally induced phase difference is generated between them.

와

로 발사되도록 광학섬유 루우프(16)에 결합된다. 이와 마찬가지로, 광파(W₂)는 각각의 전계진폭

와

로 각각의 모우드 i와 j를 발사하도록 결합된다. 프러스(+)와 마이너스(-)의 표시는 전파(傳播)방향을 나타내는 것으로서, 루우프(16) 주위에서의 시계방향은 프러스(+)로 표시되어 있고, 루우프(16) 주위에서 시계 반대방향은 마이너스(-)로 표시되어 있다.As shown in FIG. 2, the light wave W ₁ has an electric field amplitude where the modes i and j are respectively

Wow

Coupled to the optical fiber loop 16 to be fired into. Similarly, the light waves W ₂ are each field amplitude

Wow

Are combined to fire each of the modes i and j. The signs of the pruses (+) and minus (-) indicate the direction of propagation, the clockwise direction around the loop 16 is indicated by the prus (+), and the counterclockwise direction around the loop 16 is It is marked with a minus (-).

는 루우프(16)을 통과하는 동안 모우드 i에 남게되는 직선통과 성분

와, 루우프(16)을 통과하는 동안 모우드 j에 교차결합되는 교차결합 성분

로 나뉘어진다. 이와 마찬가지로,

는 성분

와

로 나뉘어지고,

는 성분

와

로 나뉘어지며,

는 성분

와

로 나뉘어진다.As the light in each mode i and j passes through the optical fiber loop 16, energy is coupled between the modes, so each electric field is divided into two components, the "straight-through" component represented by the subscript s and the subscript c. It is divided into the "cross-linked" components indicated. therefore,

Is the straight-through component that remains in the mode i while passing through the loop 16

And a crosslinking component crosslinked to the mode j while passing through the loop 16.

Divided into Similarly,

The ingredients

Wow

Divided into

The ingredients

Wow

Divided into

The ingredients

Wow

Divided into

와

로 구성되고, 단자(C")에서의 광선은 성분

와

로 구성되며, 단자(D')에서의 광선은 성분

와

로 구성되고, 단자(D")에서의 광선은 성분

와

로 구성된다(제3도 참조). 이러한 8개의 전계성분들은 결합기(14)에 의해 재결합되어서, 광학적 출력신호(W0)을 형성한다. 이 분야에 숙련된 기술자들은, 일반적으로 2개의 전계성분, 예를 들어

와

를 중첩시키면 다음 식으로 정의되는 최종 세기(Ⅰ)가 검출기(20)에 의해 측정됨을 알 수 있을 것이다.After the light wave passes through the optical fiber loop 16, the light rays at terminal C '

Wow

And the light beam at terminal C "

Wow

And the light rays at the terminal D '

Wow

Light rays at the terminal D "

Wow

(See Figure 3). These eight field components are recombined by the combiner 14 to form an optical output signal W0. Those skilled in the art will generally find two field components, for example

Wow

It can be seen that when superimposed of the final intensity (I) defined by the following equation is measured by the detector 20.

In the above specific example,? Is the phase difference between the electric field components E + is and E + ic.

과

은 안정상태 또는 "직류항"이고, 마지막 항은 전계

와

사이의 위상차 ø에 따라 변하는 크기를 가진 "간섭항"이다.The first two terms of equation (1), i.e.

and

Is the steady state or "direct current" term, the last term is an electric field

Wow

It is an "interference term" with a magnitude that varies with the phase difference ø.

,

,

,

,

,

,

및

가 상호 간섭하면, 위상에 무관한 8개의 직류항과 위상에 종속된 28개의 간섭항으로 구성된 광학 세기가 검출기(20)(제1도)에 나타난다. 위상 종속항의 결합수는 n(n-1), 즉 이 경우에는 56개로 된다. 그러나, 이 항들의 1/2은 나머지 1/2과 순서만 다르기 때문에, 결국 조합수는 28개로 되는 것이다.Generally, the eight fields

,

,

,

,

,

,

And

When the interference interferes with each other, an optical intensity composed of eight DC terms independent of phase and 28 interference terms dependent on phase appears in the detector 20 (FIG. 1). The number of couplings of the phase dependent term is n (n-1), that is, 56 in this case. However, half of these terms differ only in order from the other half, resulting in 28 combinations.

Eight DC terms are shown in FIG. 4 with one vector sum I _dc , and 28 interference terms are shown in FIG. 4 with one vector I _i . These vectors I _dc , I _{i are} shown on the complex plane. When the optical fiber loop 16 (FIG. 1) rotates, the phase dependency vector I _i rotates according to the pager method at an angle equal to the rotationally induced phase difference ø _s due to the Sagnak effect. When the projection of the interference vector I _i with respect to the real axis is added to the vector I _dc , the optical output signal W ₀ having the total optical intensity I _DET is measured at the detector 20 (FIG. 1). Curve 52 in FIG. 5 shows this optical intensity I _DET as a function of the Sagnak phase difference [Delta] _S.

As described above with respect to FIG. 2, the crosslinking between the modes i and j generates a non-rotationally induced phase difference between the above mentioned electric field components by irreversibly equalizing the optical fiber loops 16, It produces an accumulated phase error ø _e that is indistinguishable from the rotationally induced Sagnak phase difference ø _s . The phase error ø _e causes the phaser Ii to be rotated, for example, from the solid line position in FIG. 4 to the dotted line position. Accordingly, the curve 52 of FIG. 5 is moved by ø _e from the solid line position of FIG. 5 to the dotted line position, for example.

와

의 사이, 그리고

와

사이의 간섭에 의하여서는 위상오차가 나타나지 않음을 이해하기 바란다(왜냐하면, 이들 성분으로 표시되는 광선은 교차결합되지 않고 1개의 모우드로 루우프를 통과하기 때문이다). 그러나, 나머지 26개의 간섭항은 축적된 위상오차 ø_e를 가질 수 있다. 이들 26개의 간섭항은 3개의 그룹, 즉 다음과 같은 그룹 Ⅰ, 그룹 Ⅱ 및 그룹 Ⅲ으로 분류될 수 있는 26쌍의 전계성분들에 대응한다.To eliminate or reduce the accumulated phase error ø _e , it is necessary to analyze the 28 interference terms caused by the superposition of the eight field components described with reference to FIG. 2. First, electric field component

Wow

Between and

Wow

Understand that no phase error is caused by the interference between them (because the light rays represented by these components do not cross-couple and pass through the loop into one mode). However, the remaining 26 interference terms may have an accumulated phase error ø _e . These 26 interference terms correspond to 26 pairs of field components that can be classified into three groups, namely Group I, Group II and Group III as follows.

Although only the interference field components are listed and the interference terms themselves are not listed, the interference term for each component pair listed above can be easily calculated according to the example provided in Equation (1).

와, 모우드 j에서 발생되나 루우프(16)을 통과하는 동안에 모우드 i에 교차결합된 교차결합 성분

로 이루어진다. 통상적으로, 이 성분들은 식(1)에서 기술한 바와 같이 상호 간섭한다.Group I includes field component pairs that occur in different modes and are in the same mode when passing through loop 16 and then reaching combiner 14. For example, the first pair of components in group I is a straight-through component that occurs in mode i and remains in mode i while passing through loop 16.

And cross-linking components occurring in mode j but crosslinked to mode i while passing through loop 16

Is made of. Typically, these components interfere with each other as described in equation (1).

와

사이의 위상차가 무작위이어서 검출기(20)에서 0으로 평준화되기 때문에, 이들 성분 사이의 평균간섭은 0으로 된다. 이와 마찬가지로, 나머지 성분들(예를 들면

와

,

와

등) 사이의 간섭도 0으로 평준화된다. 따라서, 상술한 바와 같은 비간섭성 과원(10)을 이용함으로써, 그룹 Ⅰ에 열거된 성분들 사이의 간섭 및 이러한 간섭에 의해 생긴 위상오차들을 감소하거나 제거할 수 있다.However, since the phase difference between incoherent light rays is random, the interference between incoherent light rays is leveled to zero at the detector 20. Thus, the interference terms of group I can be eliminated by firing each beam with light that is incoherent (ie, randomized in phase with respect to the light in the other mode). Thus, for example, firing mode i as a ray that is incoherent to the ray in mode j, e.g.

Wow

Since the phase difference between them is random and equalized to zero in the detector 20, the average interference between these components becomes zero. Similarly, the remaining components (eg

Wow

,

Wow

Etc.) is also equalized to zero. Thus, by using the non-coherent orchard 10 as described above, it is possible to reduce or eliminate the interference between the components listed in group I and the phase errors caused by such interference.

The non-coherence needed to reduce the phase errors of group I is a function of the optical path length difference between the modes. If the interference length of the light beam from the light source is less than the length difference of the optical path between the two predetermined modes, the average interference and phase error between the group I components for these two modes is reduced. In the case of N modes, the phase error decreases when the interference length of the light source is smaller than the difference between the longest optical path and the shortest optical path. However, in order to completely eliminate the error in group I, the interference length must be smaller than the minimum value of the optical path length difference between the modes.

The relationship between the accumulated phase error ø _e (I) and the interference length due to the terms in Group I is based on all possible pairs of mode pairs (including the basic mode as well as the generalized polarization mode) whose interference length is greater than the optical path length difference. It can be estimated by determining the number of bonds. If this number is K, then the phase error? _E (I) is as follows.

(2)

(2)

Where N is the number of modes including both the base mode and the generalized polarization mode.

For example, in the case of an optical fiber with 3,000 modes (N = 3000), the number of bonds in the optical path length would be N (N-1) or about (3000) ² , making 9,000,000 possible bonds. If the length difference of the optical paths during these coupling is larger than the interference length of the light source,

(3)

(3)

It becomes

Therefore, the error rate due to the error of group I in this example is only 10-2 radians. For most practical applications, the magnitude of the total phase error øe should be less than this value (eg 10-2 radians). Substituting this value into equation (2) gives the following:

K≤0.01N ² (4)

Therefore, the interference length of the light source 10 should be selected to satisfy the equation (4). However, the shorter the interference length, the smaller the phase error of the group I is generally.

It is known to those skilled in the art that the typical dispersion data provided by the manufacturer of the optical fiber can be used to measure or calculate the optical path length of the optical fiber mode.

는 모우드 j내의 성분

와 쌍을 이루게 된다. 모우드 i와 모우드 j는 직교하며 직교 모우드의 전계들은 간섭하지 않기 때문에, 그룹 Ⅱ내의 항들 사이에는 간섭이 없게 된다. 그러나, 그룹 Ⅱ내의 각 쌍의 전계들의 패턴들은 "전체적"으로 보았을 때에만 직교하는 것임을 명심해야 한다. 즉, 간섭을 제거시키기 위하여서는 모든 전계패턴들은 광학섬유의 축에 수직인 평면에서 공간적으로 평준화되어야 한다. 만약 이러한 공간 평준화가 전계패턴의 일부에 대해서만 이루어진다면, 직교성이 존재하지 않게 된다. 모우드(예컨대 모우드 i와 j)의 실질적으로 모든 전계패턴들이 공간적으로 확실히 평준화되게 하기 위해서, 본 발명에서는 상술한 바와 같이 광학섬유(19)의 단부에서 빠져나온 모든 광선을 포획하기에 충분히 큰 표면적을 갖고 있는 검출기(20)을 이용하고 있다.Group II includes field component pairs that are in different modes after passing through loop 16, regardless of the original mode of the field component. Thus, for example, the electric field component in mode I

Is the component in mode j

Paired with. Since mode i and mode j are orthogonal and the electric fields of the orthogonal mode do not interfere, there is no interference between the terms in group II. However, it should be borne in mind that the patterns of each pair of electric fields in group II are orthogonal only when viewed "whole". That is, in order to eliminate interference, all electric field patterns should be spatially leveled in a plane perpendicular to the axis of the optical fiber. If this spatial leveling is done for only a part of the electric field pattern, there is no orthogonality. In order to ensure that substantially all field patterns of the modes (e.g., modes i and j) are spatially surely leveled, the present invention provides a surface area large enough to capture all the light rays exiting the ends of the optical fiber 19 as described above. The detector 20 which has is used.

와

의 중첩으로 인해 생기고, 다른 간섭항은 성분

와

의 중첩으로 인해 생긴다. 다시 말해서, 각각의 간섭항은 한 쌍의 성분, 즉 제1모우드에서 발생되어 루우프(16)을 통과하는 동안에 제2모우드에 교차결합되는 하나의 성분과, 동일한 제1모우드에서 발생되어 동일한 제2모우드에 교차결합되기는 하지만 루우프(16)을 통과하는 방향이 반대인 또 하나의 성분으로 인해 생긴다. 이들 2개의 간섭항은 주변 요소에 매우 민감하며, 사그낙 위상차보다 큰 위상오차를 발생시킬 수 있다.From the field component pairs listed in Group III there are only two interference terms, one interference term

Wow

Resulting from the superposition of, other interference terms

Wow

Occurs due to overlapping In other words, each interference term is a pair of components, one component that is generated in the first mode and cross-coupled to the second mode while passing through the loop 16, and is generated in the same first mode and is the same second. It is caused by another component that is cross-linked to the mode but is reversed in direction through the loop 16. These two interference terms are very sensitive to the surrounding elements and can produce a phase error larger than the Sagnak phase difference.

와

사이의 간섭은 다음과 같은 위상 종속항을 만든다.

Wow

The interference between them makes the following phase dependent terms:

Similarly, the interference between E + jc and E-jc also produces the following phase-dependent terms.

는 모우드 i와 j사이에 결합되는 전계에너지의 부분(fraction)이고,

은 모우드 i와 j 사이에 결합되는 광학세기의 부분이며ø_s는 이들 2개의 성분 사이의 회전식으로 유도된 사그낙 위상차이고, ø_p는 단자(C")와 단자(D') 사이에서 1개 모우드로부터 다른 모우드로 교차결합되는 광선의 전체적인 축적 위상이며, ø_q는 단자(C')와 단자(C") 사이에서 1개 모우드로부터 다른 모우드로 교차결합되는 광선의 전체적인 축적 위상이다.here,

Is the fraction of the field energy coupled between modes i and j,

Is the portion of optical intensity coupled between modes i and j, ø _s is the rotationally induced Sagnak phase difference between these two components, and ø _p is one mode between terminal (C ") and terminal (D ') Is the overall accumulation phase of the light ray crosslinked from one mode to the other, and ø _q is the overall accumulation phase of the light ray crosslinked from one mode to another between terminal C 'and terminal C ".

The vectors corresponding to these interference terms 5 and 6 are shown as vectors 56 and 58, respectively, in the complex plane of FIG. The interference terms 5 and 6 are projections on the real axis of the vectors 56 and 58, respectively. By adding vectors 56 and 58 vectorically, a composite vector 60 (Fig. 7) is produced. For simplicity of explanation, it should be understood that in Figs. 6 and 7 it is assumed that the Sagnak phase difference? _S is zero. Vector 60, as shown in the seventh degree are there separated by a phase angle ø _e (Ⅲ) from the real axis, a phase angle ø _e (Ⅲ) is due to the interference between the components of group Ⅲ induced in a non-rotary The phase error is shown. The projection on the real axis of the vector 60 is represented as follows as the algebraic sum of the two interference terms 5 and 6.

(7)

(7)

Since the detector 20 only measures components on the real axis of the vector 60, the output of the detector 20 is proportional to the logarithmic sum 7. Therefore, an error corresponding to the phase error? _E (III) (Fig. 7) occurs at the output of the detector 20.

The algebraic sum of the interference terms 7 may be expressed differently as follows.

(8)

(8)

| E _i | ² | E _j | ^{When 2} is the same, this logarithm sum 8 becomes as follows.

(9)

(9)

In this form, the effect of the change in the size of ø _p- ø _q can be distinguished from the rotationally induced Sagnak phase difference ø _s . This can be seen more easily with reference to Figs. 8 and 9, which show the effect of vectors 56 and 58 having the same size on the composite vector 60. Figs. In other words, ø _p -ø synthetic vector 60, regardless of the size of _q is therefore always be on the real axis, the direction of the composite vector 60 is independent of the size change of ø _p -ø _q. Although this change causes the size of the vector 60 to change, this change in the size of the vector 60 does not substantially affect the output of the detector 20, as described below. The change in size of 60 is equalized with the corresponding size change in the other modes of the multimode optical fiber, so that all of these changes add up to zero.

Thus, if the intensity of the light beam in each mode is emitted with a light beam having the same intensity as that of the other mode, it is possible to eliminate the phase errors due to the group III terms. Preferably, the beams in the modes should be the same in intensity until the group III interference term has a magnitude and phase angle suitable for eliminating phase error in detector 20 until it reaches combiner 14 and is dispersed into a reverse propagation wave. do. In addition, since the intensity of the mode cannot be the same if the mode does not have any light intensity, all phases must be fired until the light is scattered in the combiner 14 because the phase error of group III is made. Of course, even if all the modes are not fired, as long as the intensity in some of the multiple modes is the same, the Group III phase errors will also be reduced. As described above, the present invention uses a light emitting diode (LED) to equally distribute the optical intensity among all the optical fiber modes.

The analysis can be applied with the same result (ie elimination of phase error) for all bonding pairs of the mode in the multimode optical fiber. In addition, when multiple modes are used, the peripheral sensitivity of the detected optical signal is reduced. In this regard, it can be seen from the above description with reference to FIGS. 4 and 5 that the overall optical intensity IDET as measured by the detector 20 becomes as follows.

I _DET = I _dc + I _i cosøs (10)

Assuming that light beams with the same amplitude electric field E are emitted to the modes, I _dc is simply expressed as follows.

(11)

(11)

Where N is the number of optical fiber modes including both the base mode and the generalized polarization mode.

Furthermore, assuming that the light in each mode is incoherent for all other modes and that the detector 20 has sufficient surface area to ensure overall orthogonality, the interference terms of group I and group II become zero, The interference term I _i cosø _s of Equation (10) can be expressed as

(12)

(12)

는 k번째 모우드로부터 u번째 모우드로 결합되거나 혹은 u번째 모우드로부터 k번째 모우드로 결합된 광학전력의 부분이다.

는 교차결합되지 않은 상태로 소정의 모우드(예를 들어, k번째 모우드)에 남아 있는 이러한 광학전력의 비결합 부분이다.here,

Is the portion of optical power coupled from the kth mode to the uth mode or from the uth mode to the kth mode.

Is the uncoupled portion of this optical power that remains in a given mode (e.g., k-th mode) without crosslinking.

는 (1) 하나의 모우드 k, u에서 발생되어 다른 모우드 k, u에 교차결합되어 프러스(+) 방향으로 루우프(16)을 통과하는 광선과, (2) 동일한 하나의 모우드 k, u에서 발생되어 동일한 다른 모우드 k, u에 교차결합되지만 다른 방향 즉 마이너스(-) 방향으로 루우프(16)을 통과하는 광선과의 전체적인 축적 위상에 있어서의 차이이다. 모우드 i, j(제2도)에 관련된 상술한 경우에, 위상각

는 ø_p-ø_q의 크기와 동일하다.Also,

Is generated in (1) one mode k, u and cross-linked to another mode k, u and passes through loop 16 in the direction of prus (+), and (2) occurs in the same mode k, u This is the difference in the overall accumulation phase with the light beams passing through the loop 16 in the other direction, i. In the case described above in relation to the mode i, j (Fig. 2), the phase angle

Is equal to the size of ø _p -ø _q .

및

)에 관련된 간섭항들의 합계로 인해 나타나는 광학세기이다. 상술한 바와 같이 이들 "직선통과" 성분에 의해 위상오차가 나타나지는 않지만, 그럼에도 불구하고 이 성분들은 회전식으로 유도된 "사그낙" 위상차에 응답하여 간섭한다. 식(12)의 우변의 나머지 항은 그룹 Ⅲ 성분들에 관련된 간섭항들의 합계로 인해 나타나는 광학세기이다[(식(9)와 식(12)를 비교하여 보기 바란다]. 모든 모우드들이 동일한 세기를 갖고 있는 한, 이러한 간섭항들에 의해 위상오차가 나타나지는 않는다.The first term on the right side of equation (12) is the "straight-through" light wave component (e.g.

And

Optical intensity due to the sum of the interference terms associated with Although the phase error is not exhibited by these "straight through" components as described above, these components nevertheless interfere in response to the rotationally induced "Sagnak" phase difference. The remaining term on the right side of Eq. (12) is the optical intensity due to the sum of the interference terms associated with the Group III components (see comparison between Eq. (9) and (12).) As far as possible, no phase error is caused by these interference terms.

Equation (12) may be expressed differently as follows.

(13)

(13)

Where I _T = N | E | ²

과 η의 변화가 평준화됨으로써 더욱 안정한 신호를 만들게 된다. 신호의 안정도는 1/ N에 비례한다.Equation (13) shows that both the "straight through" interference term and the "cross-coupled" interference terms in group III are summed over N modes and divided by N, so these terms would appear to be equalized for the N modes. Can be. If there are multiple modes, due to the perturbations of the optical fiber

The changes in and η are leveled off, creating a more stable signal. The stability of the signal is proportional to 1 / N.

In addition, the phase angle ø _ku has a value between 0 and 360 °, which can be positive or negative. Therefore, as the number of modes increases, the mean value of cosø _ku converges to zero, so that equation (13) decreases as follows.

(14)

(14)

Actually,

(15)

(15)

ego,

I _dc 1 / 2I _T (16)

I found out.

Substituting Eq. (15) into Eq. (14) and substituting Eq. (14) and Eq. (15) in Eq. (10) is as follows.

I _DET = 1 / 2I _T (1 + 1 / 2cosø _s ) (17)

에 비례한다.Therefore, the intensity of the optical output signal W0 as measured by the detector 20 changes in response to the rotationally induced sagnak phase difference ø _s as in the curve 52 indicated by the solid line in FIG. . Eliminating the phase errors in groups I, II, and III results in an overall accumulated phase error ø _e of zero, so that the curve 52 remains stable with respect to the phase (e.g., does not move along the ø _s axis of FIG. 5). ). In addition, because the remaining interference terms are leveled across the N modes of the optical fiber, the amplitude of curve 52 remains stable and therefore insensitive to changes in birefringence caused by peripheral elements. The stability of the amplitude of the curve 52 is 1 /

Proportional to

The rotation sensor of the present invention also advantageously reduces the effects of light scattering back within the optical fiber loops. In this regard, it is understood that existing optical fibers are not optically complete and have defects that scatter small amounts of light. This phenomenon is commonly referred to as Rayleigh scattering. This scattering is slightly insignificant, although light rays are lost from the optical fiber, so this loss is relatively small. A fundamental problem associated with Rayleigh scattering relates to the portion of the beam that is reflected such that the beam propagates through the optical fiber in a direction opposite to the original direction of propagation. This is commonly called "backscattering" light. If such backscattered light is coherent between the light rays moving in the same direction around the loop 16, the backscattered light will interfere constructively or destructively with the light beam and thus the detector 20 The intensity of the optical output signal W ₀ to be measured is changed. In the present invention, the above-mentioned interference is reduced by using the light source 10 which can launch each mode with light rays which are incoherent with respect to the other modes. Therefore, backscattered light generated in one mode (eg, mode i) and captured by another mode (eg, mode j) does not interfere with light in this other mode (eg, mode j). In addition, when light waves are recombined at the combiner 14, for example, the light rays in the mode i are generated in the mode i and do not interfere with the backscattered light captured by another mode (e.g., the mode j), which causes Because. Eventually, the only backscattered light that can cause interference is the backscattered light that is generated in some mode and remains in the same mode while passing through the loop 16. In practice, the backscattered light is leveled across the N modes, so the amount of interference from the backscattered light is inversely proportional to the number of modes. Therefore, in order to reduce backscattering, it is preferable to use optical fibers having a large number of modes.

In the second embodiment of the present invention shown in FIG. 1 and FIG. 10, a bi-directional passing through a single mode (i.e. one of the generalized polarization modes within a particular basic mode) and rejecting light in all other modes We are using a modal filter. In a preferred embodiment of the present invention, a modal filter is a coupler (12), so configured as a transfer hologram (70) disposed in the optical path of the optical fiber 11 between of 14, the input optical wave (W _i) is a filter It is moved to the rope 16 through 70, and the output light wave W ₀ is moved to the detector 20 through the same filter 70. Referring to FIG. 10, the filter 70 is placed in the optical path of the optical fiber 11 by cutting the continuous strand of the optical fiber 11 at a predetermined position between the combiner 12, 14 Two optical fiber portions 11a, 11b. Filter 70 is disposed between the optical fiber portion (11a), (11b) to block the input wave (W _i) and the output wave (W _0).

Those skilled in the art will appreciate that such holographic filter 70 can be made by using the technique shown in FIG. As shown in this figure, in this method, a pair of multimode optical fibers 80, which are the same as the optical fiber portions 11a and 11b, except that the length is very short (e.g., 10 cm). , (82).

The pair of optical fibers 80, 82 and holographic plate 84 are relatively disposed in the exact manner as required by the optical fiber portions 11a, 11b and the filter 70. A predetermined single mode of the pair of optical fibers 80, 82 (i.e., the mode to be carried by the filter 70) is a pair of light waves W _a , (orientated to propagate towards the plate 84) ( W _b ) being emitted, the light rays emitted from the optical fibers 80 and 82 are blocked by the holographic plate 84. In the illustrated embodiment, the optical fibers 80, 82 are oriented so as to cover the same area of correctly-wave (W _a), (W _b) that impinges upon the plate from opposite sides plate 84.

After exposing the holographic plate 84 in this manner, the plate is developed and positioned between the optical fiber portions 11a and 11b to provide a filter 70. During the manufacture of the filter 70, the filter 70 and the optical fiber portions 11a, 11b in exactly the same way as the holographic plate 84 and the pair of optical fibers 82, 84 were correctly positioned. ) Relatively accurately, the filter 70 passes only the desired single mode and rejects all other modes.

Removing the phase error using the modal filter 70 can be understood in more detail by referring to the conceptual model of the modes i and j as described above with reference to FIG. Assume that filter 70 passes light in mode i and rejects another mode (eg, mode j). Thus, the input optical wave (W _i) is, after having passed through the filter 70, the optical fiber portion (11b) into the (first 10 degree) field components E _i only. Since the cross coupling between the modes occurs while passing through the loop 16, when the reverse carriers are recombined at the combiner 14 to form the optical output signal W ₀ , the following field component pairs are applied. Only the corresponding interference terms will be present.

및

만이 남게 된다. 이 성분쌍들 사이의 간섭은 위상오차에 영향을 미치지 않으므로, 검출기(20)에 의해 측정된 광학적 출력신호(W₀)에는 위상오차가 없게 된다.When the optical output signal (W _i) is transmitted to the optical fiber portion (11a) through the filter 70 from the optical fiber (11b) (FIG. No. 10), all of the components attached to the subscript j is removed interference components

And

Only remains. Since the interference between these component pairs does not affect the phase error, there is no phase error in the optical output signal W ₀ measured by the detector 20.

In this second embodiment, since only one mode of the multi-mode optical fiber is used, the optical output signal W ₀ in the second embodiment is significantly reduced in intensity compared with the case of the first embodiment. The embodiment also has the advantage that it is possible to use a relatively inexpensive multimode optical fiber as in the first embodiment.

Claims (26)

A light source means for generating light waves, a multi-mode optical fiber for maintaining a plurality of basic modes for the light waves and forming a loop for detecting rotation according to the sagnak effect, and a pair of light waves propagating in opposite directions around the loops. Means for coupling the light waves to the loop to provide an optical output signal having an intensity proportional to the phase difference between the pair of light waves passing through the loop, and a sagnak effect. And means for reducing the non-rotationally induced phase difference between the pair of light waves to detect the phase difference induced by the multiple mode optical fiber rotation sensor. 2. The apparatus of claim 1, further comprising means for detecting the optical output signal to determine the rotational speed of the loop, wherein the optical signal detected by the detection means is from all modes of the multimode optical fiber. Rotation sensor composed of light rays. The rotation sensor according to claim 1, wherein the light source means emits a plurality of modes of the optical fiber, and each mode of the multi-mode optical fiber is fired with a light beam which is substantially incoherent to light emitted into another mode. . 4. The light source according to claim 3, wherein N is the number of propagation modes of the multimode optical fiber with respect to the light source means, and K is the number of mode pairs in which the length difference of the optical path in the loop is longer than the interference length of the light beam. A rotation sensor wherein the interference length of the light rays generated by the means is such that K &lt; 0.01 N ² . 4. The rotation sensor according to claim 3, wherein said light source means comprises a light emitting diode. 4. A rotation sensor according to claim 3, wherein said light source means comprises means for phase modulating light rays generated by said light source means. 7. A rotation sensor according to claim 6, wherein said phase modulating means is arranged at a random frequency. 7. The rotational sensor of claim 6, further comprising means for detecting the optical output signal to determine the rotational speed of the loop, wherein the phase modulating means is adapted to be driven at a frequency outside the bandwidth of the detection means. 7. The rotation sensor according to claim 6, wherein said phase modulation means is disposed between said light source means and said coupling means. The rotation sensor according to claim 1, wherein said coupling means is composed of an optical fiber directional coupler. The rotation sensor according to claim 1, wherein said reduction means is constituted by a detector which substantially blocks the optical output signal. The rotation sensor according to claim 1, wherein the light waves from the light source means are emitted into the plurality of modes of the optical fiber such that the light rays in each of the plurality of modes have substantially the same intensity. 13. The rotation sensor according to claim 12, wherein said light source means comprises a light emitting diode. 13. The rotation sensor according to claim 12, wherein said light source means comprises a mode mixing circuit (40) for equalizing each intensity of light rays in said plurality of modes. 2. An optical path according to claim 1, wherein said reducing means comprises a modal filter, a first optical fiber portion for providing an optical path between said light source means and said modal filter, and an optical path between said modal filter and said coupling means. And a multimode optical fiber further forming a second optical fiber portion for providing the first optical fiber portion, wherein the first and second optical fiber portions comprise the optical means such that the modal filter blocks the light wave and the optical output signal. Interoperating with the modal filter to provide an optical path between the coupling means, wherein the modal filter includes only one base mode of the plurality of base modes of the optical fiber and only one within the base mode. Rotation sensor that is intended to pass light from only the generalized polarization mode of and block light from all other modes. 16. The rotation sensor according to claim 15, wherein said modal filter consists of holograms. Multi-mode optics consisting of a light-emitting diode and multi-mode optical fibers forming loops for sensing rotation by maintaining a number of basic modes for light rays generated by the light-emitting diodes coupled to provide light to the optical fibers. Fiber rotation sensor. 18. The apparatus of claim 17, further comprising: means for combining the light waves to couple light rays from the diode and form an optical output signal to provide a pair of light waves propagating in the reverse direction around the loop; And further comprising a detector arranged to be of a suitable size to block all. Combining a pair of light waves to propagate in a reverse direction through a multi-mode optical fiber loop formed of a multi-mode optical fiber holding a plurality of basic modes for a pair of light waves, and inducing a phase difference between the light waves according to the sagnak effect. Rotate the multimode optical fiber rou, combine the light waves to form an optical output signal, apply the optical output signal to a detector, and have a surface area large enough to block substantially all of the optical output signal. Reducing the non-rotationally induced phase difference between the backward propagation waves by selecting a detector. Combining a pair of light waves to propagate through a multimode optical fiber loop formed of multimode optical fibers retaining a plurality of basic modes for a pair of lightwaves, and fired into different modes of the multiple modes of the multimode optical fiber. Firing each of the plurality of modes of the multimode optical fiber with light that is substantially incoherent to light, thereby reducing the non-rotationally induced phase difference between the light waves and detecting the phase difference between the reverse propagation waves. Rotation detection method of a sagnak interferometer which is comprised. 21. The method of claim 20, wherein N is the number of optical fiber modes constituting the multimode optical fiber loop, and K is K <0.01 N when the length difference of the optical paths in the loop is longer than the interference length of light rays. A rotation detection method of a sagnak interferometer, further comprising selecting a light source having an interference length of ² . Combining a pair of light waves to propagate in reverse through an optical fiber loop formed of multi-mode optical fibers that maintain multiple base modes for the pair of light waves, and substantially equal in intensity of the light in the mode of the multi-mode optical fibers And reducing the non-rotationally induced phase difference between the pair of light waves. 23. The method of claim 22, wherein said reducing step comprises combining light rays from a light emitting diode to said optical fiber. 23. The method of claim 22, wherein said reducing step consists in coupling the light beam from the light source to the optical fiber and passing the light beam from the light source through a mode mixing circuit. Combining a pair of light waves to propagate through a multi-mode optical fiber loop formed of multi-mode optical fibers retaining multiple basic modes for a pair of light waves, and (1) different modes of multiple modes of the multi-mode optical fiber The pair by firing each of the plurality of modes of the multimode optical fiber with a beam that is substantially incoherent to the light emitted therein, and (2) making the intensity of the light in the modes of the multimode optical fiber substantially equal. Reducing the non-rotationally induced phase difference between the light waves of the Sagnak interferometer. 26. The method of claim 25, wherein the reducing step comprises selecting a detector having a surface area large enough to block substantially all of the optical output signal and placing the detector to block substantially all of the optical output signal. Rotation detection method of the Sagnak interferometer further comprising.

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